Summary:
Deposit Type
The most important ore deposits of the Eastern Cordillera are polymetallic hydrothermal deposits mined principally for Sn, W, Ag and Zn, with sub-product Pb, Cu, Bi, Au and Sb. They are related to stocks, domes and volcanic rocks of Middle and Late Miocene age (22 to 4 Ma). Mineralization occurs in veins, fracture swarms, disseminations and breccias.
The Bolivar deposit is considered a “Bolivian-type” polymetallic deposit which has the primary reference and quoted as described in Arce-Burgao (2009). The Bolivian vein deposits can be identified into three subgroups:
1. Deposits associated with tin porphyries;
2. Deposits associated with volcanic domes and sub volcanic stocks; and
3. Deposits associated with sedimentary rocks. This classification is based mainly on host rock lithology.
Mineralogy of the Bolivar zinc-tin deposit includes sphalerite, galena, cassiterite, jamesonite, pyrite, arsenopyrite and marcasite in a dominant gangue of quartz.
Geology
The Bolivar Mine is located in the Cordillera de los Azanaques, forming the western edge of the Cordillera Oriental, which is detached from the Cordillera de los Frailes, belonging to the group of central mountain ranges. Characterized by the essence of undulating plateaus, outstanding mountains parallel to the course of the Andes, with elevations that vary between 3,400 and 4,600 masl. The area is part of the polymetallic belt of the altiplano and the Cordillera Occidental.
The Bolivar zinc-silver-tin deposit is located in a polymetallic zone, approximately 7 km northeast of the Chualla Grande porphyritic stock. A series of predominantly east-west and northeast trending veins up to 2,000 m long are hosted by Silurian shales, sandstones and quartzites. The veins vary in width between 0.40 m and 4.50 m. The Bolívar vein, which is one of the main veins, has an average width of 1.25m, with a dip of 50º to 75º with a strike of 30º to 50º and is Zn-AgPb rich. The Pomabamba vein, which towards the SW shows a degeneration of zinc and lead and a considerable increase in silver due to the presence of silver sulphides, has approximate widths of 3.0 m to 5.00 m with a dip of 35º to 60º and a general heading from 60º to 70º. The Nané vein, which has an average width of between 0.50 m to 2.50 m, with a dip of 40º to 60º and strike of 50º, has significant Zn-Ag-Pb mineralization, although silver is reduced towards the NE. The veins have a length of up to 1,800 m for the combined Pomabamba, Nané and Bolívar composite vein. In addition, they are mined up to Level 400 (3,600 masl) and over a vertical distance of >600 m and are delineated by drilling to Level 620 (3,400 masl) for more than 1,000 m of vertical extent.
The mineralogy of the deposit is sphalerite, galena, cassiterite, jamesonite, pyrite, arsenopyrite and marcasite in a gangue of quartz. The Bolivar paragenesis is an early, low sulfidation mesothermal Zn assemblage of quartz-pyrite-Fe-rich sphalerite, with a later intermediate sulfidation epithermal Zn-Sn-Ag-Pb-Sb assemblage of sulfides and sulfosalts including low Fe sphalerite, microscopic cassiterite, and Pb-Sb sulfosalts along with jamesonite, frankeite, teallite, tetrahedrite, and late-stage galena-siderite-quartz.
The fluids from the mineralizing events generated physical-chemical exchange and reactions during mineral deposition. Alteration halos are observed mainly at the contact between the mineralized structures and the surrounding lithologies. With the silicification present mainly in the mineral structures or in the immediate caisson rock, while argillite and chlorite are abundant adjacent to the mineralized structures.
This district is typical of the Bolivian polymetallic vein-type deposits including a genetic relationship with a Miocene felsic intrusion, even though the deposit is sediment-hosted, fault control of the veins, zoned hydrothermal alteration, and a multiphase, zoned, polymetallic, and telescoped mineral paragenesis.
Mineralization
The Bolivar system is a network epigenetic hydrothermal base metal type veins and faults filled mineralization hosted within a variety of lithologies from volcanic tuffs to sedimentary packages. The main mineral assemblages are composed of sphalerite, marmatite, galena, silver-rich galena and silver sulfosalts. The resources are usually based on multiple structures containing several veins. The typical dimensions of these structures are ~500 m in length and ~450 m in depth with mineralization continuing to be open at depth with vein widths of between 0.2 m - 4.0 m.
The occurrence of a mineral deposit is related to two primordial aspects: 1) a hot intrusive body generating mineralizing fluids and 2) a pre-mineral geological structure receiving mineralization. The non-presence of an intrusive body very close to the deposit, makes one conclude that its formation is due to the influence of the Chualla Grande Stock, with minerals of higher temperature in its vicinity such as: 1) coarse cassiterite accompanied by quartz and tourmaline (at Totoral and Avicaya); 2) an intermediate or transitional zone with minerals of Fe-Sn (Buenos Aires, San Francisco, Venus) and; 3) an external zone where Bolívar is located with minerals of Zn-Pb-AgSn.
The Pomabamba mineralization corridor has a simplified mineral paragenesis of sphalerite – pyrite – sulfosalt type of Ag-Pb-Sn that differs from the Rosario mineralization corridor whose paragenesis is sphalerite – galena – pyrite – siderite. This allows one to conclude that there is a lateral zone in the mineralization that corresponds to the central part of the deposit termed the Pomabamba corridor.
The Pomabamba vein has its own characteristics longitudinally, with a predominance of marmatite-pyrite mineralization in its northern sector and abundant pyrite in the south. Vertically and at depth the pyrite becomes more dominant and the marmatite subordinate. A remarkable aspect is that pyrite is associated or is intergrown with Ag minerals mainly to the south. Another aspect to note is that, at higher levels, there is a band of brown sphalerite that can be distinguished within the marmatite-pyrite association, whose longitudinal inlay had no preferential location.
The mineralogical characteristics of the Nané vein differ from that of Pomabamba, with predominant brown sphalerite, and galena sulfosalts in smaller proportions and generally as much sphalerite and pyrite with subordinate marmatite at depth.
The Bolivar vein, which is an extension in the north direction of the Nané, presents as sphalerite (brown), sulfosalts of Pb-Sb-Ag-Sn, marmatite and pyrite, which is enriched in Ag content as a result, characteristic of its south and center sector. However, in the north, the pyrite becomes predominant and the sphalerite-sulfosalts subordinate.
The polymetallic mineralization in the Bolivar deposit according to the paragenesis concludes that it would have formed in different phases or mineralization events with a clear telescopic deposition:
• An early phase would comprise the mineral association of quartz – pyrite – sphalerite (of the marmatite type);
• Sphalerite (brown) – jamesonite – boulangerite – needle-type cassiterite – galena – franckeitite would correspond to the intermediate phase of mineralization; and
• Finally, the second-generation carbonates and quartz corresponding to the late phase mineralization.
The composition and events of the mineralization illustrate that the deposit was formed from hydrothermal solutions under intermediate temperature conditions of 250º - 300ºC, and that it classifies as a meso- to epithermal hydrothermal deposit.
The Bolivar Deposit is currently composed of seventeen (17) mineralized structures with varying geometric, structural and mineralogical characteristics. These veins have been identified and developed for more than 590 m at Levels 0 and -380.